1. Field of the Invention
This invention relates to weighing systems and, more particularly, to an on-board system for measuring the weight of a vehicle.
2. Description of the Prior Art
Strain gauges have been used to measure load induced deflection of various components of vehicle suspensions. For example, U.S. Pat. No. 4,042,049 describes a vehicle weighing system in which respective strain measuring transducers are mounted on a pair of equalizing beams positioned on opposite sides of the vehicle. The beams are pivotably secured to the vehicle frame at their centers, and they have tandem axles secured to their ends. The strain transducer measures the load induced deflection of the equalizing beams thereby providing an indication of vehicle weight.
Because of the widespread usage of leaf spring suspensions, it is extremely advantageous to devise a system capable of measuring load induced strain of leaf springs employed in this type of suspension. One skilled in the art initially looking at the problem would undoubtedly conclude that the optimum location for measuring strains would be between the clamp and one of the hangers since this is the area of greatest response to loading and it is free of interference from the leaf spring mounting structure. Mounting the strain measuring device beneath the axle clamp would not be considered desirable since it is generally felt that spring strain beneath the clamp is relatively insignificant. The strain distribution within the clamped region is not well understood and common spring design employs empirical relations to determine the effective active length of the spring. Also, it is thought that expansion of the axle clamp structure, generally a pair of U-bolts, as well as creep in a low friction liner positioned between the clamp and spring would cause the preload of the spring to vary thereby producing erroneous readings.
Contrary to conventional wisdom that the mid-point between the axle clamp and one of the hangers is the optimum location for measuring spring strain, it has been determined that this location is far from ideal. The basic problem is that the lower surface of the hangers which contact the upper surface of the springs are curved so that the distance from the axle clamp to the point of contact between the hangers and the springs varies as the vehicle load varies. This characteristic prevents the strain in the vehicle spring from being a linear function of vehicle load so that the output of a strain measuring device mounted on the spring between the axle clamp and one of the hangers would not be directly proportional to vehicle load. Also, most of the settling that takes place over the life of the spring occurs between the end and middle of the spring. This causes offsets that cannot be compensated for by strain gauges. Thus, it has not heretofore been possible to accurately measure the load carried by a vehicle employing a leaf spring suspension by utilizing on-board weighing systems.